Hide summary functionality behind feature flag (#472)

Since summaries are only used by Janus, the Just ecosystem testing tool,
it makes sense to hide it behind a feature flag.

Cargo.toml

@@ -11,6 +11,7 @@ edition     = "2018"
 [features]
 default = []
 help4help2man = []
+summary = []
 
 [dependencies]
 ansi_term      = "0.11"

src/lib.rs

@@ -60,4 +60,5 @@ mod warning;
 
 pub use crate::run::run;
 
+#[cfg(summary)]
 pub mod summary;